Nuclear magnetic resonance tomography is employed, among other things, to obtain spectroscopic information or image information about a given substance. A combination of nuclear magnetic resonance tomography with the techniques of magnetic resonance imaging (MRI) provides a spatial image of the chemical composition of the substance.
Magnetic resonance imaging is, on the one hand, a tried and true imaging method that is employed clinically worldwide. On the other hand, magnetic resonance imaging constitutes a very important examination tool for industry and research outside the realm of medicine as well. Examples of applications are the inspection of food products, quality control, pre-clinical testing of drugs in the pharmaceutical industry or the examination of geological structures, such as pore size in rock specimens for oil exploration.
The special strength of magnetic resonance imaging lies in the fact that very many parameters have an effect on nuclear magnetic resonance signals. A painstaking and controlled variation of these parameters allows experiments to be performed that are suitable to show the influence of the selected parameter.
Examples of relevant parameters are diffusion processes, probability density distributions of protons or a spin-lattice relaxation time.
In nuclear resonance tomography, atom nuclei having a magnetic momentum are oriented by a magnetic field applied from the outside. In this process, the nuclei execute a precession movement having a characteristic angular frequency (Larmor frequency) around the direction of the magnetic field. The Larmor frequency depends on the strength of the magnetic field and on the magnetic properties of the substance, particularly on the gyromagnetic constant γ of the nucleus. The gyromagnetic constant γ is a characteristic quantity for every type of atom. The atom nuclei have a magnetic momentum μ=γ×p wherein p stands for the angular momentum of the nucleus.
In nuclear resonance tomography, a substance or a person to be examined is subjected to a uniform magnetic field. This uniform magnetic field is also called a polarization field Bo and the axis of the uniform magnetic field is called the z axis. With their characteristic Larmor frequency, the individual magnetic momentums of the spin in the tissue precede around the axis of the uniform magnetic field.
A net magnetization Mz is generated in the direction of the polarization field, whereby the randomly oriented magnetic components cancel each other out in the plane perpendicular to this (the x-y plane). After the uniform magnetic field has been applied, an excitation field B1 is additionally generated. This excitation field B1 is polarized in the x-y plane and it has a frequency that is as close as possible to the Larmor frequency. As a result, the net magnetic momentum Mz can be tilted into the x-y plane in such a way that a transverse magnetization Mt is created. The transverse component of the magnetization rotates in the x-y plane with the Larmor frequency.
By varying the time of the excitation field, several temporal sequences of the transverse magnetization Mt can be generated. In conjunction with at least one applied gradient field, different slice profiles can be realized.
Particularly in medical research, there is a need to acquire information about anatomical structures, about spatial distributions of substances as well as about brain activity or about blood flow or changes in the concentration of deoxyhemoglobin in the organs of animals and humans.
Magnetic resonance spectroscopy (MRS) makes it possible to measure the spatial density distribution of certain chemical components in a material, especially in biological tissue.
A comprehensive presentation of echo-planar spectroscopic imaging (EPSI) can be found in the article by P. Mansfield: Magn. Reson. Med., 1, page 370, 1984.
Rapid magnetic resonance imaging (MRI), in conjunction with magnetic resonance spectroscopy (MRS), allows an examination of local distributions of metabolic processes. For instance, regional hemodynamics involving changes in the blood volumes and blood states as well as changes in the metabolism can be determined in vivo as a function of brain activity; in this context, see S. Posse et al.: Functional Magnetic Resonance Studies of Brain Activation; Seminars in Clinical Neuropsychiatry, Volume 1, No. 1, 1996; pages 76 to 88.
NMR imaging methods select slices or volumes that yield a measuring signal under the appropriate emission of high-frequency pulses and under the application of magnetic gradient fields; this measuring signal is digitized and stored in a one-dimensional or multi-dimensional field in a measuring computer.
A one-dimensional or multi-dimensional Fourier transformation then acquires (reconstructs) the desired image information from the raw data collected.
A reconstructed tomograph consists of pixels, and a volume data set consists of voxels. A pixel (picture element) is a two-dimensional picture element, for instance, a square. The image is made up of pixels. A voxel (volume pixel) is a three-dimensional volume element, for instance, a right parallelepiped. The dimensions of a pixel are in the order of magnitude of 1 mm2, and those of a voxel are in the order of magnitude of 1 mm3. The geometries and extensions can vary.
Seeing that, for experimental reasons, it is never possible to assume a strictly two-dimensional plane in the case of tomographs, the term voxel is often employed here as well, indicating that the image planes have a certain thickness.
Due to the large differences in the signal intensity of individual chemical substances and due to movements of an object being measured, localization artifacts can occur during the imaging and spectroscopy.
Particularly in examinations of the brain, it is necessary to suppress signals from substances that are located outside of the brain but that are located inside a slice to be examined. In the case of magnetic resonance with protons (1H), these are substances, for example, lipids, that contain 1H.
Lipids cover a rather broad frequency range that coincides with that of most metabolites. In spectroscopic examinations of the brain, it is advantageous to suppress signals from substances that are located outside of the brain but that are located inside of the slice to be examined—also referred to as lipid suppression because the signals thus generated can be much stronger than the signals in the brain regions to be examined.
In view of the fact that the lipids in the human head are primarily to be found in the periphery of the skull, one possibility of lipid suppression is to not at all excite the nuclear spins in the periphery. A spatially localized spectrum is obtained by means of signal suppression in regions located outside of a volume to be examined. Such techniques are referred to as single-voxel techniques.
A known single-voxel technique called STEAM is described in the following articles:    Garnot J. (1986): Selected volume excitation using stimulated echoes (VEST). Applications to spatially localized spectroscopy and imaging; J. Magn. Reson. 70: pages 488 to 492;    Kimmich R., Hoepfel D. (1987): Volume selective multipulse spin echo spectroscopy. J. Magn. Reson. 72: pages 379 to 384;    Frahm J., Merboldt K. D., Haenicke W. (1987): Localized proton spectroscopy using stimulated echoes. J. Magn. Reson. 72: pages 502 to 508.
Another volume localization method involving a single-voxel technique called PRESS is disclosed in U.S. Pat. No. 4,480,228 by Bottomley P. A. (1984) titled “Selective volume method for performing localized NMR spectroscopy”.
Another known volume localization method involving a single-voxel technique is presented by Ordidge R. J., Bendall M. R., Gordon R. E., Conelly A.: “Volume selection for in-vivo biological spectroscopy” in the book titled “Magnetic Resonance in Biology and Medicine”, published by Govil, Khetrapal and Saran, New Delhi, India, Tate McGraw-Hill Publishing Co. Ltd., page 387 (1985). In comparison to spectroscopic imaging, the known single-voxel techniques have the drawback that an examination of the spatial distribution of chemical substances is only possible to a limited extent. Another disadvantage of the known methods is that the signal suppression outside of a target volume is limited due to imperfections in the slice selection, whereby only a slight lipid suppression is achieved and/or whereby it is possible to select only rectangular target volumes. Particularly in the case of short echo times, it is difficult to avoid interferences from signals of peripheral lipids that have a short relaxation time T2.
It is a known procedure to select long echo times in order to reduce the effect of lipid impurities.
Examples of embodiments can be found in the following articles:    Frahm J., Bruhn H., Gyngell M. L., Merboldt K. D., Haenicke W., Sauter R. (1989): Localized high-resolution proton NMR spectroscopy using stimulated echoes. Initial application to human brain in vivo. Magn. Reson. Med.: pages 79 to 93;    Frahm J., Bruhn H., Haenicke W., Merboldt K. D., Mursch K., Markakais E. (1991): Localized proton NMR spectroscopy of brain tumors using short-echo time STEAM sequences. J. Comp. Assist. Tomogr.: 15 (6), pages 915 to 922;    Moonen C. T. W., Sobering G., van Zijl P. C. M., Gillen J., von Kienlin M., Bizzi A. (1992): Proton spectroscopic imaging of human brain. J. Magn. Reson., 98 (3): pages 556 to 575.
The article by Adalsteinsson E., Irarrazabal P., Spielman D. M., Macovski A. (1995) titled “Three-Dimensional Spectroscopic Imaging with Time-Varying Gradients”; Magn. Reson. Med., 33: pages 461 to 466, describes three-dimensional spectroscopic imaging with lipid suppression by global inversion of the signal utilizing differences in the longitudinal relaxation between individual chemical substances.
An improved water and lipid suppression by means of spectral-selective dephasing pulses is known as the BASING technique. A description of the BASING technique can be found in the article by Star-Lack J., Nelson S. J., Kurhanewicz J., Huang R., Vigneron D. (1997) titled “Improved water and lipid suppression for 3D PRESS CSI using RF Band-Selective Inversion with Gradient Dephasing (BASING)”. Magn. Reson. Med. 38: pages 311 to 321.
The BASING method comprises a frequency-selective refocusing pulse in conjunction with immediately preceding and following gradient pulses having opposite signs, which leads to dephasing.
Functional nuclear magnetic resonance makes it possible to detect dynamic changes and thus to observe processes over the course of time.
With functional magnetic resonance imaging (fMRI), images are generated that show the local changes.
It is also a known procedure to employ functional nuclear magnetic resonance, that is to say, functional nuclear magnetic resonance imaging, to examine neuronal activation. Neuronal activation is manifested by an increase of the blood flow into activated regions of the brain, whereby a drop occurs in the concentration of deoxyhemoglobin. Deoxyhemoglobin (DOH) is a paramagnetic substance that reduces the magnetic field homogeneity and thus accelerates signal relaxation. Oxyhemoglobin displays a magnetic susceptibility corresponding essentially to the structure of tissue in the brain, so that the magnetic field gradients are very small over a boundary between the blood containing oxyhemoglobin and the tissue. If the DOH concentration decreases because of a brain activity that triggers an increasing blood flow, then the signal relaxation is slowed down in the active regions of the brain. It is primarily the protons of hydrogen in water that are excited. The brain activity can be localized by conducting an examination with functional NMR methods that measure the NMR signal with a time delay (echo time). This is also referred to as susceptibility-sensitive measurement. The biological mechanism of action is known in the literature under the name BOLD effect (Blood Oxygen Level Dependent effect) and, in susceptibility-sensitive magnetic resonance measurements at a field strength of a static magnetic field of, for example, 1.5 tesla, it leads to increases of up to about 5% in the image brightness in activated regions of the brain. Instead of the endogenous contrast agent DOH, other contrast agents that cause a change in the susceptibility can also be used. Here, too, it is advantageous to suppress the lipid signals. Preference is given to using a frequency-selective lipid presaturation.
The imaging method is preferably a spectroscopic echo-planar imaging method, especially a repeated two-dimensional echo-planar imaging method, consisting of the repeated application of two-dimensional echo-planar image encoding. Spatial encoding takes place within the shortest possible period of time, which is repeated multiple times during a signal drop, preferably amounting to 20 ms to 100 ms. The multiple repetition of the echo-planar encoding during a signal drop serves to depict a course of the signal drop in the sequence of reconstructed individual images.